1. Technical Field
The present invention relates to a surface acoustic wave element.
2. Description of the Related Art
A surface acoustic wave element has been used as a filter, a resonator, or the like in an electric communication apparatus such as a cell phone. A surface acoustic wave element is an electromechanical conversion element using surface acoustic waves (hereafter referred to as “SAWs”) that propagate over a surface of an elastic body (for example, see Japanese Patent No. 3205976).
A SAW element disclosed in Japanese Patent No. 3205976 includes a hard layer made of diamond, a ZnO layer (piezoelectric material layer) formed on the hard layer, a silicon dioxide layer (temperature compensation layer) formed on the ZnO layer, and an electrode (interdigital transducer (hereafter referred to as “IDT”) formed between the ZnO layer and silicon dioxide layer. The IDT includes a pair of interdigital transducers. When an electrical signal is provided to the IDT, a voltage is applied between the pair of interdigital transducers. Then, a distortion occurs on the ZnO layer between the interdigital transducers due to an inverse piezoelectric effect, generating surface acoustic waves.
A frequency f, a phase velocity Vp, and a wavelength λ of surface acoustic waves have a relation represented by a formula f=Vp/λ. The phase velocity Vp depends on such as the vibration mode of surface acoustic waves and the material or thickness of the piezoelectric material layer, temperature compensation layer, and the like. The wavelength λ depends on the vibration mode of surface acoustic waves, the interval between the interdigital transducers, and the like. As seen, the frequency f of surface acoustic waves is determined by the configuration of the SAW element. Therefore, electrical signals obtained by converting surface acoustic waves have a highly accurate frequency.
In recent years, electric communication apparatuses have been required to increase the communication rate or communication data capacity. Also, SAW elements have been required, for example, to increase the operating frequency thereof, stabilize characteristics thereof against temperature variations, and increase the conversion efficiency. As is understood from the above-mentioned formula, it is preferable to increase the phase velocity Vp or reduce the wavelength λ in order to increase the frequency. However, a reduction in the wavelength λ requires a reduction in the interval between the interdigital transducers. This will increase the manufacturing cost. For this reason, as shown in Japanese Patent No. 3205976, a method of increasing the frequency by increasing the phase velocity Vp is often used.
However, even if the technology disclosed in Japanese Patent No. 3205976 is used, characteristics other than the operating frequency may not be enhanced.
If ZnO is used as a piezoelectric material as in Japanese Patent No. 3205976, the reflection coefficient at the interdigital transducers, of surface acoustic waves generated between the interdigital transducers becomes relatively high. This is attributable to such as a large difference in hardness (Young's modulus) between the ZnO layer and interdigital transducers. If the reflection coefficient is increased, losses of surface acoustic waves are increased, resulting in reductions in the characteristics of the SAW element. Conversely, if the thickness of the interdigital transducers is reduced, the reflection coefficient is reduced. However, the interdigital transducers increase resistance thereof thereby degrading the electrical characteristics of the SAW element.
In order to avoid such a disadvantage, it is conceivably preferable to reduce the reflection coefficient by increasing Young' modulus of the piezoelectric material layer. However, if a piezoelectric material whose Young' modulus is different from ZnO's is used, the phase velocity Vp, electromechanical coupling coefficient, or the like will be changed. This will make a change in the operating frequency or conversion efficiency of the SAW element. Therefore, much effort must be expended to design a SAW element having desired characteristics.
In particular, while the characteristics of the SAW element are stabilized against temperature variations by providing a temperature compensation layer as described in Japanese Patent No. 3205976, an increase in the number of components makes it more difficult to design a SAW element. This makes it difficult to obtain a SAW element having desired characteristics. In other words, the characteristics of a SAW element cannot be enhanced.